While googling about 3D printing, I found out about these folks:
It looks like they've figured out how to 3D print glass so that it's a nice transparent solid, rather than a milky brittle thing as was the case in the past. Transparent is important, as that means the surface can be polished. In my mind, this means they could print a nice lightweight mirror blank. Maybe the blank would need to be annealed afterwards, but that's not too hard.
Has anyone heard of this group before, or know of other groups that can 3D print transparent glass?